The present invention relates to a liquid crystal display device, and more particularly to the backlight unit that guides light to the display panel.
A liquid crystal display (LCD) is a passive display that is not able to emit light by itself. Accordingly, it requires to use a source of light to project light through the display panel in order to display all sorts of information.
The source of light may be supplied by a backlight unit. The backlight unit functions as a component of liquid crystal display device to guide light to the display panel in apparatus such as liquid crystal display monitors, portable computers, and navigation systems for vehicles.
With the needs of a large sized display area and a small sized system, a backlight unit has been changed from rear-disposed lamp type in which a lamp is disposed at the rear to side-disposed lamp type in which a lamp is disposed at the side. A backlight unit of side-disposed lamp type generally includes a mold frame, a light source such as a cold cathode fluorescent tube of 23 mm in diameter, a reflecting sheet for reflecting ahead light projected from the light source, a light guide panel for guiding ahead light reflected from the reflecting sheet, a diffusing sheet for diffusing uniformly light guided through the light guide panel, a prism sheet for enhancing the luminance of light passed through the diffusing sheet, and a protecting sheet for protecting the prism sheet. The reflecting sheet, light guide panel, diffusing sheet, prism sheet and protecting sheet are mounted in grooves of the mold frame. The light guide panel is a plate-shaped body made of transparent materials such as acrylic resin, and has the shape of which thickness at the portion thereof remote from the light source is getting gradually smaller than that of the portion thereof adjacent to the light source.
In order to fix the reflecting sheet, light guide panel, diffusing sheet, prism sheet and protecting sheet to the grooves of the mold frame, method using an adhesive or method forming light guide projections and the other grooves respectively in the light guide panel and in left and right sides of the mold frame to fit the light guide projections to the grooves has been used. However, in the latter case the light guide projections have right-angled corners, reflection and transmission of light are repeated in space between the light guide panel and the mold frame so that light may be concentrated at the right-angled corners of the light guide projections of the light guide panel. As a result of this concentration, the light guide projections of the guide panel may be brighter compared to other portions. The concentrated light may result in a poor quality LCD.
To solve this problem, there has been proposed a conventional backlight unit as disclosed in U.S. Pat. No. 5,988,827. In the backlight unit, corners of light guide projections remote from a lamp are rounded. Grooves of a mold frame corresponding to the shape of the rounded corners of the light guide projections are also rounded. By virtue of this structure, the rounded corners of the light guide projections can efficiently prevent light from being concentrated. However, it is still possible that right-angled recess portions between the light guide projections and a body of a light guide panel can be cracked when an LCD module is shaken or impacted. FIG. 1 is a perspective view illustrating a conventional light guide panel cracked by impact in direction Y. When the light guide projections is impacted by shaking of the LCD module, stress is concentrated on point P in the light guide panel. The reason is that stress at point P is about 27% larger than that at point Q. If stress at point P exceeds the yield stress, a crack 14 can be developed from point P. The crack 14 may have the light guide panel and the display panel render a poor image, resulting in a poor quality LCD. Also, if the light guide panel is excessively cracked, the light guide projections can be disengaged from the grooves such that the light guide panel can be slid to the lamp and damage it.
Accordingly, it needs an improved backlight unit having an anti-impact light guide panel for the LCD module for a mobile apparatus such as portable computers and personal digital assistants (PDAs) that are frequently exposed to impacts by falling or external forces compared to other display devices.
It is an object of the present invention to provide improved backlight units for liquid crystal display devices.
It is another object of the present invention to provide light guide panels and backlight units that can prevent impact forces by falling or external forces from cracking portions of LCD such as light guide projections and light sources and also reduce or eliminate light concentration at the portions of the LCD, thereby improving the quality of the LCD.
These and other objects are provided, according to the present invention, by a light guide panel comprising a plate-shaped body, light guide projections for fitting the light guide panel disposed on left and right sides of the plate-shaped body, and a plurality of patterns formed on a lower surface of the plate-shaped body to uniformly project light, wherein upper and lower surfaces of the light guide projections are disposed remote as much as predetermined intervals, respectively from planes that are extended from upper and lower surfaces of the plate-shaped body. It is preferable that each of light guide projections further include an inclined surface on a side thereof adjacent to a light source. The inclined surface may effectively prevent impact forces in direction Y from developing a crack at point P or Q in a right-angled recess portion of the light guide projections as shown in FIG. 1. Preferably, each of light guide projections has at least one rounded corner in order to efficiently prevent light from being concentrated. Alternatively, each of light guide projections has at least one chamfered corner.
In particular, a backlight unit of the invention includes a mold frame having grooves formed in left and right sides thereof and a light source such as a cold cathode fluorescent tube disposed on lower side thereof, a light guide panel having a plate-shaped body and light guide projections disposed on left and right sides of the plate-shaped body, and a plurality of sheets. Each of light guide projections has a thickness smaller than that of the plate-shaped body. The light guide panel is mounted on the mold frame so that the light guide projections thereof are fit to the grooves. A plurality of sheets comprises a reflecting sheet, a diffusing sheet, prism sheets, and a protecting sheet. Each of light guide projections further includes an inclined surface on a side thereof adjacent to the light source. Preferably, each of light guide projections has at least one rounded or chamfered corner in order to efficiently prevent light concentration.
The backlight unit of the invention may be used to form various systems with liquid crystal display devices. The liquid crystal display device comprises a housing, a display panel disposed in the housing, a backlight unit disposed behind the display panel, and a top chassis combined with a mold frame of the backlight unit to fixedly hold components of the backlight unit and the display panel.
Thus, the present invention can reduce the chances that external forces of falling or bending crack portions of the LCD such as the light guide projections, by making the light guide projections thinner than that of the light guide panel and forming the inclined surface on the light guide projections.
The present invention also can reduce or eliminate light concentration at the portions of the LCD by rounding or chamfering the corners of the light guide projections.